This invention relates generally to the field of variable geometry turbochargers and, more particularly, to a variable geometry turbocharger that is specially constructed to provide internal bypass exhaust gas flow for purposes of eliminating the need for an external wastegate.
Turbochargers for gasoline and diesel internal combustion engines are devices known in the art that are used for pressurizing or boosting the intake air stream, routed to a combustion chamber of the engine, by using the heat and volumetric flow of exhaust gas exiting the engine. Specifically, the exhaust gas exiting the engine is routed into a turbine housing of a turbocharger in a manner that causes an exhaust gas-driven turbine to spin within the housing.
The exhaust gas-driven turbine is mounted onto one end of a shaft that is common to a radial air compressor mounted onto an opposite end of the shaft and housed in a compressor housing. Thus, rotary action of the turbine also causes the air compressor to spin within a compressor housing of the turbocharger that is separate from the turbine housing. The spinning action of the air compressor causes intake air to enter the compressor housing and be pressurized or boosted a desired amount before it is mixed with fuel and combusted within the engine combustion chamber.
In a turbocharger it is often desirable to control the flow of exhaust gas to the turbine to improve the efficiency or operational range of the turbocharger. Variable geometry turbochargers (VGTs) have been configured to address this need. A type of such VGT is one having a variable exhaust nozzle, referred to as a variable nozzle turbocharger. Different configurations of variable nozzles have been employed to control the exhaust gas flow. One approach taken to achieve exhaust gas flow control in such VGTs involves the use of multiple pivoting vanes that are positioned annularly around the turbine inlet. The pivoting vanes are commonly controlled to alter the throat area of the passages between the vanes, thereby functioning to control the exhaust gas flow into the turbine.
Turbocharged high speed diesel and gasoline engines are known to produce excess exhaust energy at high speed, when compared to the power that is demanded by the turbocharger turbine. Thus, such turbocharged engines (comprising VGTs or conventional non-variable geometry turbochargers) are known to include a wastegate valve for purposes of controlling the maximum amount of exhaust gas that is routed to the turbocharger turbine under high speed engine operating conditions.
Wastegate valves known in the art are packaged external from the turbocharger, and are typically constructed to divert or bypass exhaust gas flow exiting the engine away from the turbocharger under high speed engine operating conditions. As noted above, this bypassing of the exhaust flow to the turbocharger turbine is desired and necessary for purposes of controlling the maximum amount of boost pressure provided by the turbocharger, so as to not damage the engine.
Because such conventional wastegate valves exist and are mounted separately from the turbocharger, they present several engineering and design challenges. One such challenge relates to placement and packaging, as such wastegate valves must be constructed for attachment with an engine exhaust system, and be configured for spatially compatible placement within or adjacent the engine compartment.
Another challenge relates to proper operation, as such wastegates must be actuated, controlled, and coordinated with the operation of both the vehicle engine and the turbocharger. This task is especially complicated when dealing with a VGT, that itself includes variable geometry members that require special actuation and control. While such conventional external wastegate valves are widely used, the above-noted challenges that are inherent with these wastegate valves add both cost and complexity to the task of controlling turbocharged engine operation.
It is, therefore, desirable, that an exhaust bypass device or mechanism be constructed for use with a VGT in a manner that minimizes and/or eliminates the above-noted challenges relating to packaging and operation. It is desired that such exhaust bypass device or mechanism be constructed in such a manner as to minimize manufacturing costs, and preferably utilize existing turbocharger structures and parts. It is further desired that such exhaust bypass device or mechanism function in a manner that does not compromise, and that preferably increases, turbocharger operation efficiency when compared to using a conventional wastegate.
Variable geometry turbochargers, constructed according to the principles of this invention, comprise a turbine housing having an exhaust gas inlet and outlet, a volute connected to the inlet, and a nozzle wall adjacent the volute. A turbine wheel is carried within the turbine housing and is attached to a shaft. A plurality of movable vanes are disposed within the turbine housing adjacent the nozzle wall, and are positioned between the exhaust gas inlet and turbine wheel.
The turbine housing of such variable geometry turbocharger comprises a bypass exhaust gas flow port disposed internally therein having an inlet opening positioned upstream from the turbine wheel, and an exhaust outlet opening positioned downstream from the turbine wheel. The vanes are positioned adjacent respective bypass ports such that the inlet opening for each port is at least partially covered by a respective vane depending on vane location, e.g., when the vane is placed in a closed position. The bypass port inlet opening is exposed for facilitating bypass exhaust gas flow through the turbocharger when the respective vane is actuated or moved into an open position.
The turbine housing may also comprise a specially configured nozzle wall terminal edge to permit additional bypass exhaust gas flow within the turbocharger. In such embodiment, the nozzle wall terminal edge is modified to facilitate exhaust gas passage from the turbine housing volute to an underneath portion of the movable vanes when the vanes are in an open position. One or more of the vanes for use in this embodiment includes means for channeling or routing gas through the vane itself to the bypass exhaust port to provided increased exhaust gas flow capacity.
The vanes used with variable geometry turbochargers of this invention each comprise an inner airfoil surface oriented adjacent the turbine wheel, and an outer airfoil surface oriented opposite the inner airfoil surface. The inner and outer airfoil surfaces define a vane airfoil thickness. A vane leading edge or nose is positioned along a first inner and outer airfoil surface junction, and a vane trailing edge positioned along a second inner and outer surface junction. The vane includes a hole disposed within a first axial vane surface substantially parallel to outer nozzle wall for receiving a respective post therein that projects outwardly from the nozzle wall. Each vane also includes an actuation tab that extends from a second axial vane surface opposite from the first vane surface.
These vanes preferably have an airfoil thickness that is greater than conventional xe2x80x9cslimxe2x80x9d vanes. In an example embodiment, such preferred vanes have an airfoil thickness that is greater than about 0.16 times a length of the vane as measured between the vane leading and trailing edges.